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CHAPTER
3
Genetic Control of Cell Function
Gene Structure
Genetic Code
Protein Synthesis
Messenger RNA
Transfer RNA
Ribosomal RNA
Regulation of Gene Expression
Gene Mutations
Chromosomes
Cell Division
Chromosome Structure
Patterns of Inheritance
Definitions
Genetic Imprinting
Mendel’s Laws
Pedigree
Gene Mapping and Technology
Genomic Mapping
Linkage Studies
Dosage Studies
Hybridization Studies
Recombinant DNA Technology
Gene Therapy
DNA Fingerprinting
enetic information is stored in the structure of deoxyribonucleic acid (DNA). DNA is an extremely stable macromolecule found in the nucleus of each cell. Because of
the stable structure of DNA, the genetic information can survive the many processes of reduction division, in which the gametes (i.e., ovum and sperm) are formed, and the fertilization
process. This stability is also maintained throughout the many
mitotic cell divisions involved in the formation of a new organism from the single-celled fertilized ovum called the zygote.
G
36
Genetic Control
of Cell Function
The term gene is used to describe a part of the DNA molecule that contains the information needed to code for the types
of proteins and enzymes needed for the day-to-day function of
the cells in the body. In addition, a gene is the unit of heredity
passed from generation to generation. For example, genes control the type and quantity of hormones that a cell produces, the
antigens and receptors that are present on the cell membrane,
and the synthesis of enzymes needed for metabolism. Of the
estimated 35,000 to 140,000 genes that humans possess, more
than 5000 have been identified and more than 2300 have been
localized to a particular chromosome. With few exceptions,
each gene provides the instructions for the synthesis of a single
protein. This chapter includes discussions of genetic regulation
of cell function, chromosomal structure, patterns of inheritance, and gene technology.
GENETIC CONTROL OF CELL FUNCTION
The genetic information needed for protein synthesis is encoded in the DNA contained in the cell nucleus. A second type
of nucleic acid, ribonucleic acid (RNA), is involved in the actual
synthesis of cellular enzymes and proteins. Cells contain several types of RNA: messenger RNA, transfer RNA, and ribosomal RNA. Messenger RNA (mRNA) contains the transcribed
instructions for protein synthesis obtained from the DNA molecule and carries them into the cytoplasm. Transcription is followed by translation, the synthesis of proteins according to the
instructions carried by mRNA. Ribosomal RNA (rRNA) provides the machinery needed for protein synthesis. Transfer
RNA (tRNA) reads the instructions and delivers the appropriate amino acids to the ribosome, where they are incorporated
into the protein being synthesized.
The mechanism for genetic control of cell function is illustrated in Figure 3-1. The nuclei of all the cells in an organism
contain the same accumulation of genes derived from the gametes of the two parents. This means that liver cells contain the
same genetic information as skin and muscle cells. For this to
be true, the molecular code must be duplicated before each succeeding cell division, or mitosis. In theory, although this has
not yet been achieved in humans, any of the highly differentiated cells of an organism could be used to produce a complete,
genetically identical organism, or clone. Each particular cell type
Chapter 3: Genetic Control of Cell Function
KEY CONCEPTS
FUNCTION OF DNA IN CONTROLLING
CELL FUNCTION
■ The information needed for the control of cell struc-
ture and function is embedded in the genetic information encoded in the stable DNA molecule.
■ Although every cell in the body contains the same
genetic information, each cell type uses only a portion of the information, depending on its structure
and function.
■ The production of the proteins that control cell func-
tion is accomplished by (1) the transcription of the
DNA code for assembly of the protein onto messenger RNA, (2) the translation of the code from messenger RNA and assembly of the protein by ribosomal RNA in the cytoplasm, and (3) the delivery of
the amino acids needed for protein synthesis to
ribosomal RNA by transfer RNA.
in a tissue uses only part of the information stored in the genetic code. Although information required for the development and differentiation of the other cell types is still present,
it is repressed.
Besides nuclear DNA, part of the DNA of a cell resides in
the mitochondria. Mitochondrial DNA is inherited from the
mother by her offspring (i.e., matrilineal inheritance). Several
Deoxyribonucleic acid (DNA)
Messenger ribonucleic acid (mRNA)
Transfer ribonucleic acid (tRNA)
Ribosomal ribonucleic acid (rRNA)
Protein synthesis
37
genetic disorders are attributed to defects in mitochondrial
DNA. Leber’s hereditary optic neuropathy was the first human
disease attributed to mutation in mitochondrial DNA.
Gene Structure
The structure that stores the genetic information in the nucleus
is a long, double-stranded, helical molecule of DNA. DNA is
composed of nucleotides, which consist of phosphoric acid, a
five-carbon sugar called deoxyribose, and one of four nitrogenous bases. These nitrogenous bases carry the genetic information and are divided into two groups: the purine bases, adenine
and guanine, which have two nitrogen ring structures, and the
pyrimidine bases, thymine and cytosine, which have one ring.
The backbone of DNA consists of alternating groups of sugar
and phosphoric acid; the paired bases project inward from the
sides of the sugar molecule. DNA resembles a spiral staircase,
with the paired bases representing the steps (Fig. 3-2). A precise
complementary pairing of purine and pyrimidine bases occurs
in the double-stranded DNA molecule. Adenine is paired with
thymine, and guanine is paired with cytosine. Each nucleotide
in a pair is on one strand of the DNA molecule, with the bases
on opposite DNA strands bound together by hydrogen bonds
that are extremely stable under normal conditions. Enzymes
called DNA helicases separate the two strands so that the genetic
information can be duplicated or transcribed.
Several hundred to almost one million base pairs can represent a gene; the size is proportional to the protein product
it encodes. Of the two DNA strands, only one is used in transcribing the information for the cell’s polypeptide-building
machinery. The genetic information of one strand is meaningful and is used as a template for transcription; the complementary code of the other strand does not make sense and
is ignored. However, both strands are involved in DNA duplication. Before cell division, the two strands of the helix separate and a complementary molecule is duplicated next to
each original strand. Two strands become four strands. During
cell division, the newly duplicated double-stranded molecules are separated and placed in each daughter cell by the
mechanics of mitosis. As a result, each of the daughter cells
again contains the meaningful strand and the complementary
strand joined in the form of a double helix. This type of DNA
replication has been termed semiconservative as opposed to
conservative (Fig. 3-3).
The DNA molecule is combined with several types of protein and small amounts of RNA into a complex known as chromatin. Chromatin is the readily stainable portion of the cell nucleus. Some DNA proteins form binding sites for repressor
molecules and hormones that regulate genetic transcription;
others may block genetic transcription by preventing access of
nucleotides to the surface of the DNA molecule. A specific
group of proteins called histones are thought to control the folding of the DNA strands.
Control of cellular activity
Genetic Code
■ FIGURE 3-1 ■ DNA-directed control of cellular activity through
synthesis of cellular proteins. Messenger RNA carries the transcribed message, which directs protein synthesis, from the nucleus
to the cytoplasm. Transfer RNA selects the appropriate amino
acids and carries them to ribosomal RNA where assembly of the
proteins takes place.
The four bases—guanine, adenine, cytosine, and thymine (uracil is substituted for thymine in RNA)—make up the alphabet
of the genetic code. A sequence of three of these bases forms the
fundamental triplet code used in transmitting the genetic information needed for protein synthesis. This triplet code is called
38
Unit One: Mechanisms of Disease
Sugar-phosphate backbone
A
A
C
G
T
A
C
T
T
A
C
G
G
A
A
Bases
Transcription
B
mRNA
C U
G
C
T
U
A
U A G A A U C
T A
T C T T A G
a codon (Table 3-1). An example is the nucleotide sequence
GCU (guanine, cytosine, and uracil), which is the triplet RNA
code for the amino acid alanine. The genetic code is a universal language used by most living cells (i.e., the code for the
amino acid tryptophan is the same in a bacterium, a plant, and
Hydrogen bonds
■ FIGURE 3-2 ■ The DNA double helix
and transcription of messenger RNA
(mRNA). The top panel (A) shows the sequence of four bases (adenine [A], cytosine [C], guanine [G], and thymine [T]),
which determines the specificity of genetic information. The bases face inward
from the sugar-phosphate backbone and
form pairs (dashed lines) with complementary bases on the opposing strand. In the
bottom panel (B), transcription creates a
complementary mRNA copy from one of
the DNA strands in the double helix.
a human being). Stop codes, which signal the end of a protein
molecule, are also present. Mathematically, the four bases can
be arranged in 64 different combinations. Sixty-one of the
triplets correspond to particular amino acids, and three are stop
signals. Only 20 amino acids are used in protein synthesis in
humans. Several triplets code for the same amino acid; therefore, the genetic code is said to be redundant or degenerate. For
example, AUG is a part of the initiation or start signal and the
codon for the amino acid methionine. Codons that specify the
same amino acid are called synonyms. Synonyms usually have
the same first two bases but differ in the third base.
Protein Synthesis
Semiconservative Model
Conservative Model
original strand of DNA
newly synthesized strand of DNA
■ FIGURE 3-3 ■ Semiconservative vs conservative models of DNA
replication as proposed by Meselson and Stahl in 1958. In semiconservative DNA replication, the two original strands of DNA unwind and a complementary strand is formed along each original
strand.
Although DNA determines the type of biochemical product
that the cell synthesizes, the transmission and decoding of information needed for protein synthesis are carried out by RNA,
the formation of which is directed by DNA. The general structure of RNA differs from DNA in three respects: RNA is a
single-stranded, rather than a double-stranded, molecule; the
sugar in each nucleotide of RNA is ribose, instead of deoxyribose; and the pyrimidine base thymine in DNA is replaced by
uracil in RNA. Three types of RNA are known: mRNA, tRNA,
and rRNA. All three types are synthesized in the nucleus by
RNA polymerase enzymes that take directions from DNA. Because the ribose sugars found in RNA are more susceptible to
degradation than the sugars in DNA, the types of RNA molecules in the cytoplasm can be altered rapidly in response to
extracellular signals.
Messenger RNA
Messenger RNA is the template for protein synthesis. It is a long
molecule containing several hundred to several thousand nucleotides. Each group of three nucleotides forms a codon that
Chapter 3: Genetic Control of Cell Function
TABLE 3-1
expressed from a single gene and reduces how much DNA must
be contained in the genome.
Triplet Codes for
Amino Acids
Amino Acid
RNA Codons
Alanine
Arginine
Asparagine
Aspartic acid
Cysteine
Glutamic acid
Glutamine
Glycine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Proline
Serine
Threonine
Tryptophan
Tyrosine
Valine
Start (CI)
Stop (CT)
GCU
CGU
AAU
GAU
UGU
GAA
CAA
GGU
CAU
AUU
CUU
AAA
AUG
UUU
CCU
UCU
ACU
UGG
UAU
GUU
AUG
UAA
GCC
CGC
AAC
GAC
UGC
GAG
CAG
GGC
CAC
AUC
CUC
AAG
GCA
CGA
GCG
CGG
GGA
GGG
AUA
CUA
CUG
UUC
CCC
UCC
ACC
CCA
UCA
ACA
CCG
UCG
ACG
UAC
GUC
GUA
GUG
UAG
UGA
39
AGA
AGG
UUA
UUG
AGC
AGU
(Guyton A. [2000]. Textbook of medical physiology [10th ed., p. 28].
Philadelphia: W.B. Saunders)
is exactly complementary to the triplet of nucleotides of the
DNA molecule. Messenger RNA is formed by a process called
transcription, in which the weak hydrogen bonds of the DNA are
broken so that free RNA nucleotides can pair with their exposed
DNA counterparts on the meaningful strand of the DNA molecule (see Fig. 3-2). As with the base pairing of the DNA
strands, complementary RNA bases pair with the DNA bases.
In RNA, uracil replaces thymine and pairs with adenine.
During transcription, a specialized nuclear enzyme, called
RNA polymerase, recognizes the beginning or start sequence of
a gene. The RNA polymerase attaches to the double-stranded
DNA and proceeds to copy the meaningful strand into a single
strand of RNA as it travels along the length of the gene. On
reaching the stop signal, the enzyme leaves the gene and releases the RNA strand. The RNA strand then is processed.
Processing involves the addition of certain nucleic acids at
the ends of the RNA strand and cutting and splicing of certain
internal sequences. Splicing often involves the removal of
stretches of RNA. Because of the splicing process, the final
mRNA sequence is different from the original DNA template.
RNA sequences that are retained are called exons, and those excised are called introns. The functions of the introns are unknown. They are thought to be involved in the activation or
deactivation of genes during various stages of development.
Splicing permits a cell to produce a variety of mRNA molecules from a single gene. By varying the splicing segments of the
initial mRNA, different mRNA molecules are formed. For example, in a muscle cell, the original tropomyosin mRNA is
spliced in as many as 10 different ways, yielding distinctly different protein products. This permits different proteins to be
Transfer RNA
The clover-shaped tRNA molecule contains only 80 nucleotides, making it the smallest RNA molecule. Its function is to
deliver the activated form of amino acids to protein molecules
in the ribosomes. At least 20 different types of tRNA are known,
each of which recognizes and binds to only one type of amino
acid. Each tRNA molecule has two recognition sites: the first
is complementary for the mRNA codon and the second is for
the amino acid itself. Each type of tRNA carries its own specific amino acid to the ribosomes, where protein synthesis is
taking place; there it recognizes the appropriate codon on the
mRNA and delivers the amino acid to the newly forming protein molecule.
Ribosomal RNA
The ribosome is the physical structure in the cytoplasm where
protein synthesis takes place. Ribosomal RNA forms 60% of
the ribosome, with the remainder of the ribosome composed
of the structural proteins and enzymes needed for protein synthesis. As with the other types of RNA, rRNA is synthesized in
the nucleus. Unlike other RNAs, ribosomal RNA is produced in
a specialized nuclear structure called the nucleolus. The formed
rRNA combines with ribosomal proteins in the nucleus to produce the ribosome, which is then transported into the cytoplasm. On reaching the cytoplasm, most ribosomes become attached to the endoplasmic reticulum and begin the task of
protein synthesis.
Proteins are made from a standard set of amino acids, which
are joined end to end to form the long polypeptide chains of
protein molecules. Each polypeptide chain may have as many
as 100 to more than 300 amino acids in it. The process of
protein synthesis is called translation because the genetic code
is translated into the production language needed for protein
assembly. Besides rRNA, translation requires the coordinated
actions of mRNA and tRNA. Each of the 20 different tRNA molecules transports its specific amino acid to the ribosome for incorporation into the developing protein molecule. Messenger
RNA provides the information needed for placing the amino
acids in their proper order for each specific type of protein.
During protein synthesis, mRNA contacts and passes through
the ribosome, which “reads” the directions for protein synthesis in much the same way that a tape is read as it passes through
a tape player (Fig. 3-4). As mRNA passes through the ribosome,
tRNA delivers the appropriate amino acids for attachment to
the growing polypeptide chain. The long mRNA molecule usually travels through and directs protein synthesis in more than
one ribosome at a time. After the first part of the mRNA is read
by the first ribosome, it moves onto a second and a third. As a
result, ribosomes that are actively involved in protein synthesis are often found in clusters called polyribosomes.
Regulation of Gene Expression
Although all cells contain the same genes, not all genes are active all of the time, nor are the same genes active in all cell
types. On the contrary, only a small, select group of genes is
active in directing protein synthesis in the cell, and this group
varies from one cell type to another. For the differentiation
40
Unit One: Mechanisms of Disease
Forming
protein
Amino acid
Peptide bond
U
A
Transfer RNA
"head" bearing anticodon
U
Ribosome
A A
■ FIGURE 3-4 ■ Protein synthesis.
A messenger RNA strand is moving
through a small ribosomal RNA
subunit in the cytoplasm. Transfer
RNA transports amino acids to the
mRNA strand and recognizes the
RNA codon calling for the amino
acid by base pairing (through its
anticodon). The ribosome then
adds the amino acid to the growing polypeptide chain. The ribosome moves along the mRNA
strand and is read sequently. As
each amino acid is bound to the
next by a peptide bond, its tRNA is
released.
G
C G G
U C G A C C A U U C G A U U U C G C C A U A G U C C U U G G C C
U U
Direction of
messenger RNA
advance
Messenger RNA
Codon
process of cells to occur in the various organs and tissues of the
body, protein synthesis in some cells must be different from
that in others. To adapt to an ever-changing environment, certain cells may need to produce varying amounts and types of
proteins. Certain enzymes, such as carbonic anhydrase, are synthesized by all cells for the fundamental metabolic processes
on which life depends.
The degree to which a gene or particular group of genes is
active is called gene expression. A phenomenon termed induction
is an important process by which gene expression is increased.
Except in early embryonic development, induction is promoted by some external influence. Gene repression is a process
whereby a regulatory gene acts to reduce or prevent gene expression. Some genes are normally dormant but can be activated by inducer substances; other genes are naturally active
and can be inhibited by repressor substances. Genetic mechanisms for the control of protein synthesis are better understood in microorganisms than in humans. However, it can be
assumed that the same general principles apply.
The mechanism that has been most extensively studied is
the one by which the synthesis of particular proteins can be
turned on and off. For example, in the bacterium Escherichia coli
grown in a nutrient medium containing the disaccharide lactose, the enzyme galactosidase can be isolated. The galactosidase catalyzes the splitting of lactose into a molecule of glucose
and a molecule of galactose. This is necessary if lactose is to be
metabolized by E. coli. However, if the E. coli is grown in a
medium that does not contain lactose, very little of the enzyme
is produced. From these and other studies, it is theorized that
the synthesis of a particular protein, such as galactosidase, requires a series of reactions, each of which is catalyzed by a specific enzyme.
At least two types of genes control protein synthesis: structural genes that specify the amino acid sequence of a polypeptide chain and regulator genes that serve a regulatory function
without stipulating the structure of protein molecules. The regulation of protein synthesis is controlled by a sequence of
genes, called an operon, on adjacent sites on the same chromosome (Fig. 3-5). An operon consists of a set of structural genes
that code for enzymes used in the synthesis of a particular product and a promoter site that binds RNA polymerase and initiates transcription of the structural genes. The function of the
operon is further regulated by activator and repressor opera-
Activator Repressor
operator operator
Promoter
Inhibition
of the
operator
Operon
Structural
Gene A
Structural
Gene B
Structural
Gene C
Enzyme A
Enzyme B
Enzyme C
Substrates
Synthesized
product
(Negative feedback)
■ FIGURE 3-5 ■
Function of the operon to control biosynthesis.
The synthesized product exerts negative feedback to inhibit function of the operon, in this way automatically controlling the concentration of the product itself. (Guyton A., Hall J.E. [2000].
Textbook of medical physiology [10th ed., p. 31]. Philadelphia: W.B.
Saunders with permission from Elsevier Science)
Chapter 3: Genetic Control of Cell Function
tors, which induce or repress the function of the promoter. The
activator and repressor sites commonly monitor levels of the
synthesized product and regulate the activity of the operon
through a negative feedback mechanism. When product levels
decrease, the function of the operon is activated, and when levels increase, its function is repressed. Regulatory genes found
elsewhere in the genetic complex can exert control over an
operon through activator or repressor substances. Not all genes
are subject to induction and repression.
Gene Mutations
Rarely, accidental errors in duplication of DNA occur. These errors are called mutations. Mutations result from the substitution
of one base pair for another, the loss or addition of one or more
base pairs, or rearrangements of base pairs. Many of these mutations occur spontaneously; others occur because of environmental agents, chemicals, and radiation. Mutations may arise
in somatic cells or in germ cells (ovum or sperm). Only those
DNA changes that occur in germ cells can be inherited. A somatic mutation affects a cell line that differentiates into one or
more of the many tissues of the body and is not transmissible
to the next generation. Somatic mutations that do not have an
impact on the health or functioning of a person are called polymorphisms. Occasionally, a person is born with one brown eye
and one blue eye because of a somatic mutation. The change or
loss of gene information is just as likely to affect the fundamental processes of cell function or organ differentiation. Such
somatic mutations in the early embryonic period can result in
embryonic death or congenital malformations. Somatic mutations are important causes of cancer and other tumors in which
cell differentiation and growth get out of control. Each year,
hundreds of thousands of random changes occur in the DNA
molecule because of environmental events or metabolic accidents. Fortunately, fewer than 1 in 1000 base pair changes result in serious mutations. Most of these defects are corrected by
DNA repair mechanisms. Several mechanisms exist, and each
depends on specific enzymes such as DNA repair nucleases.
Fishermen, farmers, and others who are excessively exposed to
the ultraviolet radiation of sunlight have an increased risk for
development of skin cancer as a result of potential radiation
damage to the genetic structure of the skin-forming cells.
In summary, genes are the fundamental unit of information storage in the cell. They determine the types of proteins
and enzymes made by the cell and therefore control inheritance and day-to-day cell function. Genes store information in
a stable macromolecule called DNA. Genes transmit information contained in the DNA molecule as a triplet code. The
arrangement of the nitrogenous bases of the four nucleotides
(i.e., adenine, guanine, thymine [or uracil in RNA], and cytosine) forms the code. The transfer of stored information into
production of cell products is accomplished through a second
type of macromolecule called RNA. Messenger RNA transcribes the instructions for product synthesis from the DNA
molecule and carries it into the cell’s cytoplasm, where ribosomal RNA uses the information to direct product synthesis.
Transfer RNA acts as a carrier system for delivering the appropriate amino acids to the ribosomes, where the synthesis of
cell products occurs. Although all cells contain the same
41
genes, only a small, select group of genes is active in a given
cell type. In all cells, some genetic information is repressed,
whereas other information is expressed. Gene mutations
represent accidental errors in duplication, rearrangement,
or deletion of parts of the genetic code. Fortunately, most
mutations are corrected by DNA repair mechanisms in
the cell.
CHROMOSOMES
Most genetic information of a cell is organized, stored, and retrieved in small intracellular structures called chromosomes.
Although the chromosomes are visible only in dividing cells,
they retain their integrity between cell divisions. The chromosomes are arranged in pairs; one member of the pair is inherited from the father, the other from the mother. Each species
has a characteristic number of chromosomes. In the human,
46 single or 23 pairs of chromosomes are present. Of the
23 pairs of human chromosomes, there are 22 pairs called autosomes that are alike in males and females. Each of the 22 pairs
of autosomes has the same appearance in all individuals, and
each has been given a numeric designation for classification
purposes (Fig. 3-6).
The sex chromosomes make up the 23rd pair of chromosomes. Two sex chromosomes determine the sex of a person.
All males have an X and Y chromosome (i.e., an X chromosome from the mother and a Y chromosome from the father);
all females have two X chromosomes (i.e., one from each parent). Only one X chromosome in the female is active in controlling the expression of genetic traits; however, both X chromosomes are activated during gametogenesis (i.e., formation
of the germ cell or ovum). In the female, the active X chromosome is invisible, but the inactive X chromosome can be
demonstrated with appropriate nuclear staining. This inactive
chromatin mass is seen as the Barr body in epithelial cells or as
the drumstick body in the chromatin of neutrophils. The genetic sex of a child can be determined by microscopic study of
cell or tissue samples. The total number of X chromosomes is
equal to the number of Barr bodies plus one (i.e., an inactive
plus an active X chromosome). For example, the cells of a normal female have one Barr body and therefore a total of two
X chromosomes. A normal male has no Barr bodies. Males
with Klinefelter’s syndrome (one Y, an inactive X, plus an active X chromosome) exhibit one Barr body. In the female,
whether the X chromosome derived from the mother or that
derived from the father is active is determined within a few
days after conception; the selection is random for each postmitotic cell line. This is called the Lyon principle, after Mary
Lyon, the British geneticist who described it.
Cell Division
There are two types of cell division: mitosis and meiosis. Meiosis
is limited to replicating germ cells and takes place only once in
a cell line. It results in the formation of gametes or reproductive
cells (i.e., ovum and sperm), each of which has only a single set
of 23 chromosomes. Meiosis is typically divided into two distinct phases, meiotic divisions I and II. Similar to mitosis, cells
42
Unit One: Mechanisms of Disease
■ FIGURE 3-6 ■ Karyotype of normal human boy. (Courtesy of the Prenatal Diagnostic and Imaging
Center, Sacramento, CA. Frederick W. Hansen, MD, Medical Director)
about to undergo the first meiotic division replicate their DNA
during interphase. During metaphase I, homologous chromosomes pair up, forming a synapsis or tetrad (two chromatids per
chromosome). They are sometimes called bivalents. The X and
Y chromosomes are not homologs and do not form bivalents.
While in metaphase I, an interchange of chromatid segments
can occur. This process is called crossing over (Fig. 3-7). Crossing
over allows for new combinations of genes, increasing genetic
variability. After telophase I, each of the two daughter cells contains one member of each homologous pair of chromosomes
and a sex chromosome (23 double-stranded chromosomes).
No DNA synthesis occurs before meiotic division II. During
anaphase II, the 23 double-stranded chromosomes (two chromatids) of each of the two daughter cells from meiosis I divide
at their centromeres. Each subsequent daughter cell receives
23 single-stranded chromatids. Thus, a total of four daughter
cells are formed by a meiotic division of one cell (Fig. 3-8).
Meiosis, which occurs only in the gamete-producing cells
found in either testes or ovaries, has a different outcome in
males and females. In males, meiosis (spermatogenesis) results
in four viable daughter cells called spermatids that differentiate
into sperm cells. In females, gamete formation or oogenesis is
quite different. After the first meiotic division of a primary
oocyte, a secondary oocyte and another structure called a polar
body are formed. This small polar body contains little cyto-
KEY CONCEPTS
CHROMOSOME STRUCTURE
■ The DNA that stores genetic material is organized
into 23 pairs of chromosomes. There are 22 pairs of
autosomes, which are alike for males and females,
and one pair of sex chromosomes, with XX pairing
in females and XY pairing in males.
■ Mitosis refers to the duplication of chromosomes in
somatic cell lines, in which each daughter cell
receives a pair of 23 chromosomes.
■ Meiosis is limited to replicating germ cells and
results in the formation of a single set of
23 chromosomes.
■ FIGURE 3-7 ■
Crossing over of DNA at the time of meiosis.
Chapter 3: Genetic Control of Cell Function
43
Primordial
germ cell
2N = 4
Oogonia
duplication of chromosomes
A
sister
chromatids
B
Primary
oocyte
Secondary
oocyte
First
polar
body
centromere
Second polar body
C
Ovum
A
meiosis I
Discarded in
first polar body
B
meiosis II
N=2
■ FIGURE 3-8 ■
Discarded in
second polar body
N=2
Separation of chromosomes at the time of
C
meiosis.
plasm, but it may undergo a second meiotic division, resulting
in two polar bodies (Fig. 3-9). The secondary oocyte undergoes
its second meiotic division, producing one mature oocyte and
another polar body. Four viable sperm cells are produced during spermatogenesis, but only one ovum from oogenesis.
Chromosome Structure
Cytogenetics is the study of the structure and numeric characteristics of the cell’s chromosomes. Chromosome studies can be
done on any tissue or cell that grows and divides in culture.
Lymphocytes from venous blood are frequently used for this
purpose. After the cells have been cultured, a drug called colchicine is used to arrest mitosis in metaphase. A chromosome
spread is prepared by fixing and spreading the chromosomes
on a slide. Subsequently, appropriate staining techniques show
the chromosomal banding patterns so they can be identified.
The chromosomes are photographed, and the photomicrograph of each chromosome is cut out and arranged in pairs according to a standard classification system. The completed picture is called a karyotype, and the procedure for preparing the
picture is called karyotyping. A uniform system of chromosome
classification was originally formulated at the 1971 Paris Chro-
■ FIGURE 3-9 ■
Essential stages of meiosis in a female, with discarding of the first and second polar bodies and formation of the
ovum with the haploid number of chromosomes. The dotted line
indicates reduction division. (Adapted from Cormack D.H. [1993].
Essential histology. Philadelphia: J.B. Lippincott)
mosome Conference and was later revised to describe the chromosomes as seen in more elongated prophase and prometaphase preparations.
In the metaphase spread, each chromosome takes the form
of chromatids to form an “X” or “wishbone” pattern. Human
chromosomes are divided into three types according to the position of the centromere (i.e., central constriction; Fig. 3-10).
If the centromere is in the center and the arms are of approximately the same length, the chromosome is said to be
metacentric; if it is off center and the arms are of clearly different lengths, it is submetacentric; and if it is near one end, it
is acrocentric. The short arm of the chromosome is designated
44
Unit One: Mechanisms of Disease
Centromere
p
2
22
21
1 11
Chromatid
Metacentric
1
Chromosome
arm
13
Submetacentric
Acrocentric
21
q
Satellite
2
25
■ FIGURE 3-10 ■
Three basic shapes and the component parts
of human metaphase chromosomes. The relative size of the
satellite on the acrocentric is exaggerated for visibility. (Adapted
from Cormack D.H. [1993]. Essential histology. Philadelphia: J.B.
Lippincott)
as “p” for “petite,” and the long arm is designated as “q” for
no other reason than it is the next letter of the alphabet. Arms
of the chromosome are indicated by the chromosome number followed by the p or q designation. Chromosomes 13, 14,
15, 21, and 22 have small masses of chromatin called satellites
attached to their short arms by narrow stalks. At the ends of
each chromosome are special DNA sequences called telomeres.
Telomeres allow the end of the DNA molecule to be replicated completely.
The banding patterns of a chromosome are used in describing the position of a gene. Regions on the chromosomes are
numbered from the centromere outward. The regions are
further divided into bands and subbands, which are also numbered (Fig. 3-11). These numbers are used in designating the
position of a gene on a chromosome. For example, Xp22.2
refers to subband 2, band 2, region 2 of the short arm (p) of the
X chromosome.
28
Ichthyosis
Kallman syndrome
Ocular albinism
Hypophosphatemia, hereditary
Glycogen storage disease, hepatic
Duchenne/Becker muscular dystrophy
Chronic granulomatous disease
Retinitis pigmentosa
Wiskott-Aldrich syndrome
Menkes disease
SCID, X-linked
Testicular feminization
Agammaglobulinemia
Alport syndrome
Fabry disease
Lesch-Nyhan syndrome
Hemophilia B
Fragile X syndrome
Adrenoleukodystrophy
Color blindness
Diabetes insipidus, nephrogenic
G6PD deficiency
Hemophilia A
Mucopolysaccharidosis II
■ FIGURE 3-11 ■
The localization of representative inherited diseases on the X chromosome. Notice the nomenclature of arms
(P,Q), regions (1,2), bands (e.g., 11,22). (Rubin E., Farber J.L.
[1999]. Pathology [3rd ed., p. 260]. Philadelphia: Lippincott
Williams & Wilkins)
PATTERNS OF INHERITANCE
The characteristics inherited from a person’s parents are inscribed in gene pairs located along the length of the chromosomes. Alternate forms of the same gene are possible (i.e., one
inherited from the mother and the other from the father), and
each may produce a different aspect of a trait.
KEY CONCEPTS
TRANSMISSION OF GENETIC INFORMATION
■ The transmission of information from one genera-
In summary, the genetic information in a cell is organized, stored, and retrieved as small cellular structures called
chromosomes. Forty-six chromosomes arranged in 23 pairs are
present in the human being. Twenty-two of these pairs are
autosomes. The 23rd pair is the sex chromosomes, which determine the sex of a person. Two types of cell division occur,
meiosis and mitosis. Meiosis is limited to replicating germ
cells and results in the formation of gametes or reproductive
cells (ovum and sperm), each of which has only a single set of
23 chromosomes. Mitotic division occurs in somatic cells and
results in the formation of 23 pairs of chromosomes. A karyotype is a photograph of a person’s chromosomes. It is prepared by special laboratory techniques in which body cells are
cultured, fixed, and then stained to demonstrate identifiable
banding patterns. A photomicrograph is then made. Often
the individual chromosomes are cut out and regrouped
according to chromosome number.
tion to the next is vested in genetic material
transferred from each parent at the time of
conception.
■ Alleles are the alternate forms of a gene (one from
each parent), and the locus is the position that they
occupy on the chromosome.
■ The genotype of a person represents the sum total
of the genetic information in the cells and the
phenotype the physical manifestations of that
information.
■ Penetrance is the percentage in a population with a
particular genotype in which that genotype is phenotypically manifested, whereas expressivity is the
manner in which the gene is expressed.
Chapter 3: Genetic Control of Cell Function
Definitions
Genetics has its own set of definitions. The genotype of a person is the genetic information stored in the base sequence
triplet code. The phenotype refers to the recognizable traits,
physical or biochemical, associated with a specific genotype.
Often, the genotype is not evident by available detection methods. More than one genotype may have the same phenotype. Some brown-eyed persons are carriers of the code for
blue eyes, and other brown-eyed persons are not. Phenotypically, these two types of brown-eyed persons are the same, but
genotypically they are different.
When it comes to a genetic disorder, not all persons with a
mutant gene are affected to the same extent. Expressivity refers
to the manner in which the gene is expressed in the phenotype,
which can range from mild to severe. Penetrance represents the
ability of a gene to express its function. Seventy-five percent
penetrance means 75% of persons of a particular genotype
demonstrate a recognizable phenotype. Syndactyly (webbed
fingers) and blue sclera are genetic mutations that often do not
exhibit 100% penetrance.
The position of a gene on a chromosome is called its locus,
and alternate forms of a gene at the same locus are called alleles.
When only one pair of genes is involved in the transmission of
information, the term single-gene trait is used. Single-gene traits
follow the Mendelian laws of inheritance.
Polygenic inheritance involves multiple genes at different loci,
with each gene exerting a small additive effect in determining
a trait. Most human traits are determined by multiple pairs of
genes, many with alternate codes, accounting for some of the
dissimilar forms that occur with certain genetic disorders. Polygenic traits are predictable but less so than single-gene traits.
Multifactorial inheritance is similar to polygenic inheritance in
that multiple alleles at different loci affect the outcome; the difference is that multifactorial inheritance includes environmental effects on the genes.
Generation I
Many other gene–gene interactions are known. These include epistasis, in which one gene masks the phenotypic effects
of another nonallelic gene; multiple alleles, in which more than
one allele affects the same trait (e.g., ABO blood types); complementary genes, in which each gene is mutually dependent on
the other; and collaborative genes, in which two different genes
influencing the same trait interact to produce a phenotype
neither gene alone could produce.
Genetic Imprinting
Besides autosomal and sex-linked genes and mitochondrial inheritance, it was found that certain genes exhibit a “parent of
origin” type of transmission, in which the parental genomes do
not always contribute equally in the development of an individual (Fig. 3-12). Although rare, it is estimated that approximately 100 genes exhibit genomic, or genetic, imprinting.
Evidence suggests there is a genetic conflict over the developing
embryo: the male genome attempts to establish larger offspring, whereas the female prefers smaller offspring to conserve
her energy for the current and subsequent pregnancies.
An example of a disorder involving parenteral imprinting is
uniparental disomy. This occurs when two chromosomes of the
same number are inherited from one parent. Normally, this is
not a problem, except in cases where a chromosome has been
imprinted by a parent. If an allele is inactivated by imprinting,
the offspring will have only one working copy of the chromosome, resulting in possible problems.
Mendel’s Laws
The main feature of inheritance is predictability: given certain
conditions, the likelihood of the occurrence or recurrence of a
specific trait is remarkably predictable. The units of inheritance
are the genes, and the pattern of single-gene expression often
A
C
Generation II
■ FIGURE 3-12 ■ Pedigree of genetic imprinting. In generation I, male A has inherited a
mutant allele from his affected mother (not
shown); the gene is “turned off” during spermatogenesis, and therefore none of his offspring (generation II) will express the mutant
allele, regardless if they are carriers. However,
the gene will be “turned on” again during oogenesis in any of his daughters (B) who inherit
the allele. All offspring (generation III) who inherit the mutant allele will be affected. All offspring of normal children (C) will produce
normal offspring. Children of female D will all
express the mutation if they inherit the allele.
45
B
Generation III
D
Affected individuals
Have the mutant allele but are not affected
Do not have the mutant allele and not affected
46
Unit One: Mechanisms of Disease
can be predicted using Mendel’s laws of genetic transmission.
Techniques and discoveries since Gregor Mendel’s original
work was published in 1865 have led to some modification of
his original laws.
Mendel discovered the basic pattern of inheritance by conducting carefully planned experiments with simple garden
peas. Experimenting with several phenotypic traits in peas,
Mendel proposed that inherited traits are transmitted from parents to offspring by means of independently inherited factors—
now known as genes—and that these factors are transmitted as
recessive and dominant traits. Mendel labeled dominant factors (his round peas) “A” and recessive factors (his wrinkled
peas) “a.” Geneticists continue to use capital letters to designate
dominant traits and lowercase letters to identify recessive traits.
The possible combinations that can occur with transmission of
single-gene dominant and recessive traits can be described by
constructing a figure called a Punnett square using capital and
lowercase letters (Fig. 3-13).
The observable traits of single-gene inheritance are inherited
by the offspring from the parents. During maturation, the primordial germ cells (i.e., sperm and ovum) of both parents
undergo meiosis, or reduction division, in which the number
of chromosomes is divided in half (from 46 to 23). At this time,
the two alleles from a gene locus separate so that each germ cell
receives only one allele from each pair (i.e., Mendel’s first law).
According to Mendel’s second law, the alleles from the different gene loci segregate independently and recombine randomly in the zygote. Persons in whom the two alleles of a given
pair are the same (AA or aa) are called homozygotes. Heterozygotes have different alleles (Aa) at a gene locus. A recessive trait
is one expressed only in a homozygous pairing; a dominant trait
is one expressed in either a homozygous or a heterozygous
pairing. All persons with a dominant allele (depending on the
penetrance of the genes) manifest that trait. A carrier is a person who is heterozygous for a recessive trait and does not manifest the trait. For example, the genes for blond hair are recessive and those for brown hair are dominant. Therefore, only
persons with a genotype having two alleles for blond hair
would be blond; persons with either one or two brown alleles
would have dark hair.
Pedigree
A pedigree is a graphic method for portraying a family history
of an inherited trait. It is constructed from a carefully obtained
family history and is useful for tracing the pattern of inheritance for a particular trait.
In summary, inheritance represents the likelihood of the
occurrence or recurrence of a specific genetic trait. The genotype refers to information stored in the genetic code of a person, whereas the phenotype represents the recognizable
traits, physical and biochemical, associated with the genotype. Expressivity refers to the expression of a gene in the
phenotype, and penetrance is the ability of a gene to express
its function. The point on the DNA molecule that controls the
inheritance of a particular trait is called a gene locus. Alternate
forms of a gene at one gene locus are called alleles. The alleles
at a gene locus may carry recessive or dominant traits. A recessive trait is one expressed only when there are two copies
(homozygous) of the recessive alleles. Dominant traits are expressed with either homozygous or heterozygous pairing of
the alleles. A pedigree is a graphic method for portraying a
family history of an inherited trait.
GENE MAPPING AND TECHNOLOGY
Female
Male
Dd
Dd
D
d
D
d
DD
Dd
1/4
1/4
Dd
dd
1/4
1/4
DD=1/4
Dd=1/2
dd=1/4
■ FIGURE 3-13 ■ The Punnett square showing all possible combinations for transmission of a single gene trait (dimpled cheeks).
The example shown is when both parents are heterozygous (dD)
for the trait. The alleles carried by the mother are on the left and
those carried by the father are on the top. The D allele is dominant
and the d allele is recessive. The DD and Dd offspring have dimples and the dd offspring does not.
The genome is the gene complement of an organism.
Genomic mapping is the assignment of genes to specific chromosomes or parts of the chromosome. The Human Genome
Project, which started in 1990, is an international project to
identify and localize the estimated 35,000 to 140,000 genes
in the human genome. This is a phenomenal undertaking because there are approximately 3.2 billion base pairs in the
human genome. The national effort toward genomic mapping
is jointly coordinated by the National Institutes of Health and
the Department of Energy. Organizers of the U.S. Human
Genome Project hope to have the completed map by the year
2005. It is also anticipated that the project will reveal the
chemical basis for as many as 4000 genetic diseases. It is expected to provide tests for screening and diagnosing genetic
disorders and the basis for new treatments. To date, chromosomes 14, 20, 21 and 22 have been completely mapped.
Although the human genome project has received most of
the publicity, genome sequencing also is underway in a variety
of animals, such as the mouse and fruit fly, and other organisms, from bacteria to plants. These and other endeavors will
be of benefit in animal husbandry and agriculture and the
study of genetic similarities among various species. The human
genome project has created an anticipation of future benefits
from its undertakings as well as ethical concerns that must be
dealt with by both scientists and the public.
Chapter 3: Genetic Control of Cell Function
Genomic Mapping
Two types of genomic maps exist: genetic maps and physical
maps. Genetic maps are like highway maps. They use linkage
studies (e.g., dosage, hybridization) to estimate the distances
between chromosomal landmarks (i.e., gene markers). Physical maps are similar to a surveyor’s map. They measure the actual physical distance between chromosomal elements in biochemical units, the smallest being the nucleotide base.
Genetic maps and physical maps have been refined over
the decades. The earliest mapping efforts localized genes on
the X chromosome. The initial assignment of a gene to a particular chromosome was made in 1911 for the color blindness gene inherited from the mother (i.e., following the
X-linked pattern of inheritance). In 1968, the specific location
of the Duffy blood group on the long arm of chromosome 1
was determined. The locations of more than 2300 expressed human genes have been mapped to a specific chromosome and
most of them to a specific region on the chromosome. However, genetic mapping is continuing so rapidly that these
numbers are constantly being updated. Documentation of
gene assignments to specific human chromosomes is updated
almost daily in the Online Mendelian Inheritance in Man
(http://www.ncbi.nlm.nih.gov/Omim), an encyclopedia of
expressed gene loci. Another source is the Human Genome
Project, which is the central database for mapped genes and
an international repository for most mapping information.
Many methods have been used for developing genetic maps.
The most important ones are family linkage studies, gene
dosage methods, and hybridization studies. Often, the specific assignment of a gene is made using information from
several mapping techniques.
Most of the genome mapping has been accomplished by a
method called transcript mapping. The two-part process begins
with isolating mRNAs immediately after they are transcribed.
Next, the complementary DNA molecule is prepared. Although
transcript mapping may provide knowledge of the gene sequence, it does not automatically mean that the function of the
genetic material has been determined.
Linkage Studies
Mendel’s laws were often insufficient to explain the transmission of several well-known traits, such as color blindness and
hemophilia A. In some families it was noted that these two
conditions were transmitted together.
Linkage studies assume that genes occur in a linear array
along the chromosomes. During meiosis, the paired chromosomes of the diploid germ cell exchange genetic material because of the crossing-over phenomenon (see Fig. 3-7). This exchange usually involves more than one gene; large blocks of
genes (representing large portions of the chromosome) usually
are exchanged. Although the point at which the block separates
from another occurs randomly, the closer together two genes
are on the same chromosome, the greater the chance is that
they will be passed on together to the offspring. When two inherited traits occur together at a rate significantly greater than
would occur by chance, they are said to be linked.
Several methods take advantage of the crossing over and recombination of genes to map a particular gene. In one method,
any gene that is already assigned to a chromosome can be
used as a marker to assign other linked genes. For example, it
47
was found that an extra long chromosome 1 and the Duffy
blood group were inherited as a dominant trait, placing the
position of the blood group gene close to the extra material on
chromosome 1. Color blindness has been linked to classic
hemophilia A (i.e., lack of factor VIII) in some pedigrees;
hemophilia A has been linked to glucose-6-phosphate dehydrogenase deficiency in others; and color blindness has
been linked to glucose-6-phosphate dehydrogenase deficiency
in still others. Because the gene for color blindness is found on
the X chromosome, all three genes must be located in a small
section of the X chromosome. Linkage analysis can be used
clinically to identify affected persons in a family with a known
genetic defect. Males, because they have one X and one Y chromosome, are said to be hemizygous for sex-linked traits. Females
can be homozygous (normal or mutant) or heterozygous for
sex-linked traits. Heterozygous females are known as carriers for
X-linked defects.
One autosomal recessive disorder that has been successfully
diagnosed prenatally by linkage studies using amniocentesis is
congenital adrenal hyperplasia (due to 21-hydroxylase deficiency), which is linked to an immune response gene (human
leukocyte antigen [HLA] type). Postnatally, linkage studies
have been used in diagnosing hemochromatosis, which is
closely linked to another HLA type. Persons with this disorder
are unable to metabolize iron, and it accumulates in the liver
and other organs. It cannot be diagnosed by conventional
means until irreversible damage has been done. Given a family history of the disorder, HLA typing can determine if the gene
is present, and if it is present, dietary restriction of iron intake
may be used to prevent organ damage.
Dosage Studies
Dosage studies involve measuring enzyme activity. Autosomal
genes are normally arranged in pairs, and normally both are expressed. If both alleles are present and both are expressed, the
activity of the enzyme should be 100%. If one member of the
gene pair is missing, only 50% of the enzyme activity is present,
reflecting the activity of the remaining normal allele.
Hybridization Studies
A recent biologic discovery revealed that two somatic cells from
different species, when grown together in the same culture, occasionally fuse to form a new hybrid cell. Two types of hybridization methods are used in genomic studies: somatic cell
hybridization and in situ hybridization.
Somatic cell hybridization involves the fusion of human somatic cells with those of a different species (typically, the
mouse) to yield a cell containing the chromosomes of both
species. Because these hybrid cells are unstable, they begin to
lose chromosomes of both species during subsequent cell divisions. This makes it possible to obtain cells with different partial combinations of human chromosomes. By studying the enzymes that these cells produce, it is possible to determine that
an enzyme is produced only when a certain chromosome is
present; the coding for that enzyme must be located on that
chromosome.
In situ hybridization involves the use of a specific sequence
of DNA or RNA to locate genes that do not express themselves
in cell culture. DNA and RNA can be chemically tagged with
radioactive or fluorescent markers. These chemically tagged
48
Unit One: Mechanisms of Disease
DNA or RNA sequences are used as probes to determine gene
location. The probe is added to a chromosome spread after
the DNA strands have been separated. If the probe matches the
complementary DNA of a chromosome segment, it hybridizes
and remains at the precise location (thus the term in situ) on a
chromosome. Radioactive or fluorescent markers are used to
determine the location of the probe.
Recombinant DNA Technology
During the past several decades, genetic engineering has provided the methods for manipulating nucleic acids and recombining genes (recombinant DNA) into hybrid molecules that
can be inserted into unicellular organisms and reproduced
many times over. Each hybrid molecule produces a genetically
identical population, called a clone, that reflects its common
ancestor.
The techniques of gene isolation and cloning rely on the
fact that the genes of all organisms, from bacteria through
mammals, are based on similar molecular organization. Gene
cloning requires cutting a DNA molecule apart, modifying and
reassembling its fragments, and producing copies of the modified DNA, its mRNA, and its gene product. The DNA molecule
is cut apart by using a bacterial enzyme, called a restriction enzyme, that binds to DNA wherever a particular short sequence
of base pairs is found and cleaves the molecule at a specific
nucleotide site. In this way, a long DNA molecule can be broken into smaller, discrete fragments with the intent that one
fragment contains the gene of interest. More than 100 restriction enzymes are commercially available that cut DNA at different recognition sites.
The selected gene fragment is replicated through insertion
into a unicellular organism, such as a bacterium. To do this, a
cloning vector such as a bacterial virus or a small DNA circle
that is found in most bacteria, called a plasmid, is used. Viral
and plasmid vectors replicate autonomously in the host bacterial cell. During gene cloning, a bacterial vector and the DNA
fragment are mixed and joined by a special enzyme called a
DNA ligase. The recombinant vectors formed are then introduced into a suitable culture of bacteria, and the bacteria are allowed to replicate and express the recombinant vector gene.
Sometimes, mRNA taken from a tissue that expresses a high
level of the gene is used to produce a complementary DNA
molecule that can be used in the cloning process. Because the
fragments of the entire DNA molecule are used in the cloning
process, additional steps are taken to identify and separate the
clone that contains the gene of interest.
In terms of biologic research and technology, cloning makes
it possible to identify the DNA sequence in a gene and produce
the protein product encoded by a gene. The specific nucleotide
sequence of a cloned DNA fragment can often be identified by
analyzing the amino acid sequence and mRNA codons of its
protein product. Short sequences of base pairs can be synthesized, radioactively labeled, and subsequently used to identify
their complementary sequence. In this way, identifying normal
and abnormal gene structures is possible. Proteins that formerly were available only in small amounts can now be made
in large quantities once their respective genes have been isolated. For example, genes encoding for insulin and growth hormone have been cloned to produce these hormones for pharmacologic use.
Gene Therapy
Although quite different from inserting genetic material into a
unicellular organism such as bacteria, techniques are available
for inserting genes into the genome of intact multicellular
plants and animals. Promising delivery vehicles for these
genes are the adenoviruses. These viruses are ideal vehicles because their DNA does not become integrated into the host
genome; however, repeated inoculations are often needed because the body’s immune system usually targets cells expressing adenovirus proteins. Sterically stable liposomes also show
promise as DNA delivery mechanisms. This type of therapy is
one of the more promising methods for the treatment of genetic disorders, certain cancers, cystic fibrosis, and many infectious diseases.
Two main approaches are used in gene therapy: transferred
genes either replace defective genes or selectively inhibit deleterious genes. Cloned DNA sequences or ribosomes usually are
the compounds used in gene therapy. However, the introduction of the cloned gene into the multicellular organism can influence only the few cells that get the gene. An answer to this
problem would be the insertion of the gene into a sperm or
ovum; after fertilization, the gene would be replicated in all of
the differentiating cell types. Even so, techniques for cell insertion are limited. Not only are moral and ethical issues involved,
but these techniques cannot direct the inserted DNA to attach
to a particular chromosome or supplant an existing gene by
knocking it out of its place.
DNA Fingerprinting
The technique of DNA fingerprinting is based in part on those
techniques used in recombinant DNA technology and those
originally used in medical genetics to detect slight variations in
the genomes of different individuals. Using restrictive endonucleases, DNA is cleaved at specific regions. The DNA fragments
are separated according to size by electrophoresis (i.e., Southern blot) and transferred to a nylon membrane. The fragments
are then broken apart and subsequently annealed with a series
of radioactive probes specific for regions in each fragment. An
autoradiograph reveals the DNA fragments on the membrane.
When used in forensic pathology, this procedure is undertaken
on specimens from the suspect and the forensic specimen.
Banding patterns are then analyzed to see if they match. With
conventional methods of analysis of blood and serum enzymes, a 1 in 100 to 1000 chance exists that the two specimens
match because of chance. With DNA fingerprinting, these odds
are 1 in 100,000 to 1 million.
In summary, the genome is the gene complement of an
organism. Genomic mapping is a method used to assign
genes to particular chromosomes or parts of a chromosome.
The most important ones used are family linkage studies,
gene dosage methods, and hybridization studies. Often the
specific assignment of a gene is determined by using information from several mapping techniques. Linkage studies assign
a chromosome location to genes based on their close association with other genes of known location. Recombinant DNA
studies involve the extraction of specific types of messenger
RNA used in synthesis of complementary DNA strands. The
Chapter 3: Genetic Control of Cell Function
complementary DNA strands, labeled with a radioisotope, bind
with the genes for which they are complementary and are
used as gene probes. Now underway is an international project to identify and localize all 35,000 to 140,000 genes in the
human genome. Genetic engineering has provided the methods for manipulating nucleic acids and recombining genes
(recombinant DNA) into hybrid molecules that can be inserted into unicellular organisms and reproduced many times
over. As a result, proteins that formerly were available only in
small amounts can now be made in large quantities once
their respective genes have been isolated. DNA fingerprinting,
which relies on recombinant DNA technologies and those of
genetic mapping, is often used in forensic investigations.
REVIEW QUESTIONS
■ Explain the mechanisms by which genes control cell function
and the characteristics of the next generation.
■ Define gene locus and allele.
■ Explain the difference between a person’s genotype and
phenotype.
■ Describe the concepts of induction and repression as they
apply to gene function.
■ Describe the pathogenesis of gene mutation.
■ Explain how gene expressivity and penetrance determine the
effects of a mutant gene that codes for the production of an
essential enzyme.
■ Differentiate between autosomes and sex chromosomes and
between meiosis and mitosis.
■ Explain the significance of the Barr body.
■ Construct a hypothetical pedigree for a recessive and dominant trait according to Mendel’s law.
49
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for links to chapter-related resources on the Internet.
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